Costa is Costa after all. So it's not about the drinks being served but the ambiance.
What makes this place good is the staff. They are all friendly and do their best for the client to enjoy their "costa experience". Admittedly it does get a bit crowded sometimes especially when there are lots of pushchairs etc. However, it goes to show the staff are doing something right if it's always so busy.
As said elsewhere. It may be the only franchised coffee shop in town. But it wouldn't be so busy with repeat customers if they weren't doing something right :-)